What will you use for a heat shield/gasket for the driver's side manifold? Any gasket that you use has to be for a 273/318, not 340 or 360. Just went through this same thing with my '72 318, 340 stuff will seal the top and sides, but will leave the bottom open. That heat shield/gasket is almost impossible to find. Post #13 for your header pipes.
Good luck.